1. Field of the Invention
The present invention relates to a step-up switching regulator that controls an output voltage by an input voltage control signal. Particularly, the present invention relates to a switching regulator that controls an output rectifying circuit by monitoring the voltage control signal.
2. Description of the Related Art
FIG. 8 is a circuit diagram of a step-up switching regulator 100 according to the related art. The switching regulator 100 steps up a power supply voltage Vcc to a predetermined voltage and outputs an output voltage Vout at an output terminal OUT. The step-up switching regulator 100 performs a PWM control to control the on-duty cycle of a switching transistor MN101 such that a divided voltage Vfb obtained by dividing the output voltage Vout is equal to a predetermined reference voltage Vref.
In the example of FIG. 8, a diode D101 is used as a rectifying element. In this case, the power supply voltage Vcc is output to the output terminal OUT at the time of shut-down. This is a problem of step-up switching regulators. In order to solve the problem, a circuit configuration illustrated in FIG. 9 has been proposed. The circuit of FIG. 9 includes a synchronous-rectifying transistor MP101 as a rectifying element to which a signal synchronized with the gate voltage of a switching transistor MN101 is input. The circuit also includes a transistor MP102 connected in series with the synchronous-rectifying transistor MP101 in order to prevent the output of the power supply voltage Vcc to the output terminal OUT upon shut-down (see Japanese Laid-open Patent Publication No. 2009-178033, for example).
However, in the configuration of FIG. 9, an inductor current flows backward in a light-load status, thereby reducing efficiency. Thus, the transistor MP102 needs to be controlled by detecting the light-load status. However, the control requires a complicated circuit and is not easy to perform.